This invention relates to a method and apparatus for controlling the current supplied to an objective lens in a scanning electron microscope (SEM), or some other similar charged particle beam scanning device.
As shown in FIG. 1, a SEM or some other similar device incorporates a specimen stage 1 supporting a specimen holder 2 in which a specimen 3 is placed. The specimen 3 is irradiated by an electron beam 8 which is focused by the objective lens (final stage condenser lens) 5 energized by an objective lens power supply 7. The output current of the objective lens power supply 7 is adjusted in a focusing operation so that the electron beam 8 is focused on the specimen surface S by the objective lens 5. For various purposes, the specimen stage 1 is shifted vertically along the optical axis 4 of the objective lens 5 by means of a stage driving means 6. Heretofore, the change in vertical position of the specimen stage 1 after said focusing operation was detected by the stage driving means 6 to produce a corresponding displacement signal. The displacement signal was supplied to the objective lens power supply 7 so that a correcting excitation current component could be supplied to the objective lens 5. The correcting current component shifted the focusing plane of the electron beam 8 automatically.
The specimen surface plane S is usually different from the reference plane S' flush with the upper surface of the holder 2 as indicated by broken line in FIG. 1, particularly in the case that the specimen has large protrusions and dents. Further, the relation between the excitation current (I) of the objective lens and the focusing distance (D) (between the electron beam focusing plane and the principal plane of the objective lens) normally is not linear. Accordingly, the precise correcting excitation current value for compensating the defocus due to the specimen shift by the objective lens is not obtained simply by the information of the vertical positioning signal (corresponding to the said reference plane S') of the specimen stage 1. And it is apparent from the above reasons that the above said conventional method for automatically correcting the focus condition is extremely inaccurate. Therefore, each time the specimen stage is vertically shifted, it is necessary to readjust an excitation current of the objective lens to precisely focus the electron beam on the specimen surface, a procedure which is quite troublesome.
An object of the present invention is to provide a method and apparatus for automatically and precisely correcting the focus condition without regard to the vertical shift of a specimen, and thereby enhance the operability of the SEM or the like.